A　mathematical　model　was　developed　to　investigate　the　water　vapor　condensation　under　supersonic　flow　conditions.　A　numerical　simulation　was　performed　for　the　water　vapor　in　nozzle　with　the　model　with　various　initial　concentration　and　sizes　of　exoteric　nuclei.　It　was　found　that　the　influences　of　the　radius　of　on　the　condensation　are　different　for　different　concentration　of　the　exoteric　nuclei　and　the　effects　of　the　concentration　of　the　exoteric　nuclei　on　the　condensation　are　different　for　the　exoteric　nuclei　of　different　radius.　Therefore　it　is　of　basic　importance　to　introduce　external　particles　of　proper　sizes　and　condensation　in　order　to　enhance　the　condensation　of　steam　in　nozzle.　It　was　found　that　the　liquid　mass　fraction　at　outlet　of　nozzle　is　maximal　when　the　particle　radius　is　1.0　×　10-9　m　the　particle　concentration　is　1.0　×　1014/　kg　for　the　Laval　nozzle　and　the　condition　studied.